Normal-looking female geeks exist. Sometime in their teens they just learn to hide their geekiness (propably because of peer pressure, but I'm just guessing here.)
They will not admit their geekiness to you before they know you well and have determined you to be a fellow geek.
Also if you are both still in school of some kind, well, you're out of luck. She would get shunned by her peers were she openly associating herself with (eww) geeks.
On second thought she propably already is a near-outcast in her peer group. This might be a clue in identifying these undercover geeks.
You might look for some kind of a behaviour signature: Think how does a male proto-geek behave when he is about to awaken to full geekdom? (No, we are not geeks from birth. We quite propably are predisposed though.)
Then again, she might just be an ordinary lunatic. Be careful. ;)
Also being a geek is not an reason to being physically weak. Yes, most geek friends of mine are actually slightly overweight, but they are not in poor shape. Note that I'm not claiming that they are anything resembling body builders.
Wasn't it relatively recently when the geek stereotype was a skeletal freckled young male wearing glasses and carrying a calculator? Whatever happened to it?
Sorry for the rambling style. I just woke up and english isn't my primary language.